This application claims the priority of German patent document 199 61 312.5, filed Dec. 18, 1999, the disclosure of which is expressly incorporated by reference herein.
The invention relates to a method and apparatus for. controlling a continuously variable automatic transmission in which the transmission output torque acting upon the driving wheels of a vehicle and the engine reaction torque retroacting upon the driving engine can be adjusted by way of a regulated transmission variable.
So-called CVT transmissions (continuously variable transmissions) have continuously variable gears in which a defined and desired gear ratio can be adjusted by way of the most variable drives (such as belts and bevel gears).
Furthermore, there are continuously variable transmission gears in which transmission output torque can be adjusted by the regulated transmission variable. The transmission output torque acting upon the driving wheels determines by way of the physical transmission quantities the engine reaction torque retroacting upon the driving engine. The ratio in the transmission occurs as a function of the torque balance at the engine interface. Transmissions of the type described above are commercially available, for example, from Torotrak Company. In the invention described in the following, specifically such a continuously variable transmission gear is used which has an adjustable transmission torque.
It is an object of the present invention to provide a method and apparatus for controlling a continuously variable automatic transmission, with which a transmission regulator or moment regulator operation can easily be carried out.
This and other objects and advantages are achieved by the present invention, in which the transmission power loss is calculated, for example, by means of a physical model, and a desired transmission output power provided from a demand determination is added. On the whole, this results in a power demand of the driving engine which at the time is composed specifically of the desired transmission output power and of the transmission power loss relative to the desired operating point. By thus taking into account of the transmission power loss, starting is permitted in a system in which the power is considered.
In this case, the desired transmission output torque for providing a desired transmission output power is generated particularly from a wheel torque requirement. The power demand of the driving engine is the basis for determining the desired engine torque as well as of the desired rotational speed of the engine. Both operating point values of the engine can be calculated by means of given algorithms or can be selected by means of given values (generating of the operating point).
During operation in the torque regulator mode, a specific transmission output torque is set which causes a reaction torque on the other side of the transmission. The transmission ratio occurs as a consequence of the torque balance at the engine interface, satisfying the equilibrium condition between the engine torque provided by the engine and the engine reaction torque.
In contrast, during operation in the transmission regulator mode, the transmission control implements a specific defined ratio. In a first approximation, the driving torque will then be a result of the given engine torque; specifically a function of the transmission ratio and its losses.
In a transmission whose output torque is to be adjusted, the process and apparatus described above permit operation in the torque regulator mode, in the transmission ratio regulator mode or in a mixed operation.
During operation in the torque regulator mode, the rotational speed of the engine is regulated to a desired rotational speed which is the result of generating the operating point. According to a first control alternative, the desired engine torque is corrected as a function of the difference between the actual rotational engine speed and a desired rotational engine speed for reaching the desired rotational speed. According to a concurrent second control alternative, the regulated transmission variable is changed so as to obtain a corrected desired transmission output torque. Preferably, the first control alternative is given priority, because it requires no torque change at the transmission output. Only the engine demand is varied in order to achieve the required desired rotational speed by way of the engine torque excess or underrecovery.
If the available engine potential is not sufficient, it is necessary to reduce the desired transmission output torque and thus the engine reaction torque. This, however, results in a torque collapse at the transmission output.
In order to ensure this priority as between changing the desired engine torque and changing the desired transmission output torque during the torque regulator operation mode, it is advantageous to check whether the desired engine torque can be implemented by the driving engine. This can be done, for example, by means of a model calculation.
In contrast, during operation in the transmission ratio regulator mode, the regulated transmission variable is adjusted so that the actual transmission ratio corresponds to a calculated desired transmission ratio. In this case, the actual transmission ratio is preferably determined from the actual rotational engine and transmission speeds. The desired transmission ratio may be determined from the desired engine rotational speed and the actual rotational transmission output speed, by means of a defined algorithm, or by means of defined values.
In the transmission ratio regulator operating mode, by changing a regulated transmission variable, the actual transmission ratio can also be regulated in a targeted manner to a value which differs from the desired transmission ratio. Such deviation may be required for a brief period in order to take into account special system conditions or to implement special requirements.
Other objects, advantages and novel features of the present invention will become apparent from the following detailed description of the invention when considered in conjunction with the accompanying drawings.